U.S. patent number 10,903,435 [Application Number 16/162,696] was granted by the patent office on 2021-01-26 for flexible display device.
This patent grant is currently assigned to DONGWOO FINE-CHEM CO., LTD.. The grantee listed for this patent is DONGWOO FINE-CHEM CO., LTD.. Invention is credited to Seungwoo Lee, Inkyu Song.
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United States Patent |
10,903,435 |
Lee , et al. |
January 26, 2021 |
Flexible display device
Abstract
A flexible display device is disclosed. The flexible display
device includes a display panel; a functional structure disposed on
the display panel; a window disposed on the functional structure; a
first adhesive layer formed between the display panel and the
functional structure; and a second adhesive layer formed between
the functional structure and the window, wherein the first adhesive
layer and the second adhesive layer have a tan .delta. of 0.8 to 1
at -20.degree. C. The flexible display device is excellent in
bending resistance and durability while including a plurality of
layers.
Inventors: |
Lee; Seungwoo (Hwaseong-si,
KR), Song; Inkyu (Pyeongtaek-si, KR) |
Applicant: |
Name |
City |
State |
Country |
Type |
DONGWOO FINE-CHEM CO., LTD. |
Iksan-si |
N/A |
KR |
|
|
Assignee: |
DONGWOO FINE-CHEM CO., LTD.
(Iksan-si, KR)
|
Appl.
No.: |
16/162,696 |
Filed: |
October 17, 2018 |
Prior Publication Data
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|
|
|
Document
Identifier |
Publication Date |
|
US 20190115547 A1 |
Apr 18, 2019 |
|
Foreign Application Priority Data
|
|
|
|
|
Oct 18, 2017 [KR] |
|
|
10-2017-0135455 |
Jul 20, 2018 [KR] |
|
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10-2018-0084717 |
|
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H01L
51/56 (20130101); H01L 27/3232 (20130101); B32B
7/12 (20130101); H01L 51/0097 (20130101); H01L
51/524 (20130101); H01L 51/0001 (20130101); B32B
27/08 (20130101); C09J 183/04 (20130101); H01L
51/5293 (20130101); H01L 27/323 (20130101); H01L
51/5281 (20130101); H01L 2251/5338 (20130101) |
Current International
Class: |
H01L
51/00 (20060101); B32B 27/08 (20060101); C09J
183/04 (20060101); H01L 27/32 (20060101); B32B
7/12 (20060101); H01L 51/56 (20060101); H01L
51/52 (20060101) |
Field of
Search: |
;345/173,178 ;348/552
;257/40 ;428/189 ;361/749 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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10-2012-0076026 |
|
Jul 2012 |
|
KR |
|
10-2013-0130698 |
|
Dec 2013 |
|
KR |
|
10-2015-0084257 |
|
Jul 2015 |
|
KR |
|
10-2015-0084260 |
|
Jul 2015 |
|
KR |
|
10-1579710 |
|
Dec 2015 |
|
KR |
|
10-1645066 |
|
Aug 2016 |
|
KR |
|
Primary Examiner: Dharia; Prabodh M
Attorney, Agent or Firm: Sughrue Mion, PLLC
Claims
The invention claimed is:
1. A flexible display device comprising: a display panel; a
functional structure disposed on the display panel; a window
disposed on the functional structure; a first adhesive layer formed
between the display panel and the functional structure; and a
second adhesive layer formed between the functional structure and
the window, wherein the first adhesive layer and the second
adhesive layer each have a tan .delta. of 0.8 to 1 at -20.degree.
C., wherein the first adhesive layer and the second adhesive layer
each have a storage elastic modulus of 0.05 to 0.15 MPa at
-20.degree. C., wherein each of the first adhesive layer and the
second adhesive layer are formed from an adhesive composition
comprising an aliphatic (meth)acrylate polymer, a (meth)acrylate
ester monomer, and a photoinitiator, and wherein the (meth)acrylate
ester monomer comprises a C.sub.10-C.sub.20 alkyl
(meth)acrylate.
2. The flexible display device of claim 1, wherein the display
panel comprises an organic light emitting diode panel or a liquid
crystal display panel.
3. The flexible display device of claim 1, wherein the functional
structure comprises at least one of a touch sensor, a polarizing
layer, and a retardation layer.
4. The flexible display device of claim 1, wherein the window
comprises a substrate layer and a hard coating layer formed on at
least one side of the substrate layer.
5. The flexible display device of claim 1, wherein the
(meth)acrylate ester monomer is contained in an amount of 45 to 70
wt % based on 100 wt % of the total weight of the adhesive
composition.
6. The flexible display device of claim 1, wherein the functional
structure comprises a touch sensor, a polarizing layer, and a third
adhesive layer, wherein the third adhesive layer is formed between
the touch sensor and the polarizing layer, and wherein the third
adhesive layer has a tan .delta. of 0.8 to 1 at -20.degree. C.
7. The flexible display device of claim 1, further comprising a
protective film disposed on the window.
8. The flexible display device of claim 7, further comprising a
fourth adhesive layer formed between the window and the protective
film, wherein the fourth adhesive layer has a tan .delta. of 0.8 to
1 at -20.degree. C.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
The instant application claims priority based on Korean Patent
Application No. 10-2017-0135455 filed Oct. 18, 2017 and Korean
Patent Application No. 10-2018-0084717 filed Jul. 20, 2018, the
entire contents of which are hereby incorporated by reference.
TECHNICAL FIELD
The present relates to a flexible display device. More
specifically, the present invention relates to a flexible display
device having excellent bending resistance and durability while
including a plurality of layers.
BACKGROUND ART
Recently, the display market is rapidly changing based on flat
panel displays that are easy to fabricate over a large area and can
be reduced in weight. These flat panel displays include a liquid
crystal display (LCD), a plasma display panel (PDP), an organic
light emitting display (OLED) and the like.
A window substrate for protecting the display panel from an
external environment may be disposed on the upper part of the
display panel. The window substrate and the display panel may be
formed of, for example, a transparent plastic material having
flexibility, as the flexible display is being recently developed.
Further, additional elements of a display device such as a
polarizing plate, a touch screen panel and the like may be disposed
between the window substrate and the display panel.
For example, Korean Patent Laid-open Publication No. 2012-0076026
discloses a touch screen panel including a transparent substrate
and a polarizing plate.
However, as a plurality of layers such as a polarizing plate, a
touch screen panel, and a window substrate are laminated on a
display panel, sufficient bending resistance and durability
required for a flexible display device may not be realized.
Therefore, there is a need to develop a flexible display device
with a multi-layered structure having excellent durability while
ensuring sufficient bending resistance to prevent damage such as
cracks or breakage in each layer during bending or folding
operation.
DETAILED DESCRIPTION OF THE INVENTION
Technical Problem
It is one object of the present invention to provide a flexible
display device having excellent bending resistance and durability
while including a plurality of layers.
Technical Solution
In accordance with one aspect of the present invention, the present
invention provides a flexible display device comprising: a display
panel; a functional structure disposed on the display panel; a
window disposed on the functional structure; a first adhesive layer
formed between the display panel and the functional structure; and
a second adhesive layer formed between the functional structure and
the window, wherein the first adhesive layer and the second
adhesive layer have a tan .delta. of 0.8 to 1 at -20.degree. C.
In one embodiment of the present invention, the display panel may
comprise an organic light emitting diode (OLED) panel or a liquid
crystal display (LCD) panel.
In one embodiment of the present invention, the functional
structure may comprise at least one of a touch sensor, a polarizing
layer, and a retardation layer.
Advantageous Effects
As the flexible display device according to the present invention
has a multi-layered structure and the tan .delta. of the adhesive
layers between respective layers is controlled to be 0.8 to 1 at
-20.degree. C., the minimum bending deformation and stress are
applied to the respective layers and the durability is excellent
while ensuring sufficient bending resistance.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a cross-sectional view schematically illustrating the
flexible display device according to one embodiment of the present
invention.
FIG. 2 is a cross-sectional view schematically illustrating the
display panel of the flexible display device according to one
embodiment of the present invention.
FIG. 3 is a plan view schematically illustrating the touch sensor
of the flexible display device according to one embodiment of the
present invention.
FIG. 4 is a graph illustrating a strain profile of the flexible
display device according to one embodiment of the present
invention.
FIG. 5 is a cross-sectional view schematically illustrating the
flexible display device according to another embodiment of the
present invention.
FIG. 6 is a cross-sectional view schematically illustrating the
flexible display device according to still another embodiment of
the present invention.
DETAILED DESCRIPTION OF THE EMBODIMENTS
Hereinafter, the present invention will be described in detail with
reference to the accompanying drawings.
FIG. 1 is a cross-sectional view schematically illustrating the
flexible display device according to one embodiment of the present
invention.
Referring to FIG. 1, the flexible display device may comprise a
display panel 100, a functional structure 200 and a window 300.
The display panel 100 may comprise, for example, an organic light
emitting diode (OLED) panel or a liquid crystal display (LCD)
panel. The detailed configuration and structure of the display
panel 100 will be described below with reference to FIG. 2.
The functional structure 200 may, for example, comprise at least
one of a touch sensor, a polarizing layer, and a retardation layer.
The detailed configuration and structure of the touch sensor will
be described below with reference to FIG. 3.
The functional structure 200 may have a thickness of 50 to 150
.mu.m.
The polarizing layer may be provided, for example, as a polarizing
plate including a stretched polarizer. In this case, the polarizing
plate may include a polarizer and a protective film formed on at
least one side of the polarizer.
The stretched polarizer may include, for example, a stretched
polyvinyl alcohol (PVA)-based resin. The polyvinyl alcohol-based
resin may be preferably a polyvinyl alcohol-based resin obtained by
saponifying a polyvinyl acetate-based resin. The polyvinyl
acetate-based resin may include a copolymer of vinyl acetate and
other monomers copolymerizable therewith, in addition to polyvinyl
acetate which is a homopolymer of vinyl acetate. The other monomers
may include an unsaturated carboxylic acid-based monomer, an
unsaturated sulfonic acid-based monomer, an olefin-based monomer, a
vinyl ether-based monomer, an ammonium group-containing
acrylamide-based monomer, or the like. In addition, the polyvinyl
alcohol-based resin may be modified, and for example, it may be a
polyvinyl formal or a polyvinyl acetal modified with an
aldehyde.
The protective film may include, for example, polyester-based
resins such as polyethylene terephthalate, polyethylene
isophthalate, polyethylene naphthalate, polybutylene terephthalate
and the like; cellulose-based resins such as diacetyl cellulose,
triacetyl cellulose and the like; polycarbonate-based resins,
acryl-based resins such as polymethyl (meth)acrylate, polyethyl
(meth)acrylate and the like; cyclic olefin-based polymers (COP),
and the like.
The polarizing layer may include a coating type polarizer. In this
case, the polarizing layer may be directly coated onto the touch
sensor, and the functional structure 200 may include a laminated
structure of the touch sensor and the polarizing layer. For
example, the polarizing layer may include an alignment film and a
liquid crystal layer formed on the alignment film.
For example, an alignment film coating composition comprising an
aligning polymer, a photopolymerization initiator and a solvent is
coated and cured on the touch sensor to form an alignment film, and
then a liquid crystal coating composition is coated and cured on
the alignment film to form a polarizing layer containing an
alignment film and a liquid crystal layer.
The aligning polymer may include, for example, a polyacrylate-based
resin, a polyamic acid resin, a polyimide-based resin, a polymer
containing a cinnamate group, and the like. The liquid crystal
coating composition may comprise a reactive liquid crystal compound
and a dichroic dye.
The retardation layer may be a coating layer or a film. The
retardation layer may be provided by integration with the
polarizing layer.
The retardation layer may be a single layer or a multi-layer. In
the case of a single layer, it may be a 1/4 wave plate, and in the
case of a multi-layer, it may be a multi-layer having a 1/4 wave
plate and a 1/2 wave plate, but is not limited thereto. In the case
of the multi-layer having a 1/4 wave plate and a 1/2 wave plate,
the color sense and image quality are excellent due to phase
difference correction when applied to a display device.
The window 300 may be exposed, for example, to the user viewing
side of the flexible display device and thus be provided as a
protective film for an external environment.
The window 300 may comprise a substrate layer containing a plastic
material or a polymeric material having flexilibity. For example,
the substrate layer may include polyimide (PI), polyethersulfone
(PES), polyacrylate (PAR), polyetherimide (PEI), polyethylene
napthalate (PEN), polyethylene terephthalate (PET), polyphenylene
sulfide (PPS), polyazylate, polycarbonate (PC), cellulose
triacetate (TAC), cellulose acetate propionate (CAP) and the like.
In particular, among them, polyimide is preferable. They may be
used alone or in combination of two or more thereof.
The window 300 may further comprise a hard coating layer formed on
at least one side of the substrate layer. The hard coating layer
may be formed using a hard coating composition comprising, for
example, a photocurable compound, a photoinitiator and a solvent,
thereby further securing excellent flexibility, abrasion resistance
and surface hardness of the window 300.
The photocurable compound may include, for example, a
siloxane-based compound, an acrylate-based compound, a compound
having a vinyl group, and the like. They may be used alone or in
combination of two or more thereof.
The photoinitiator is not particularly limited as long as it
initiates a polymerization reaction of the photocurable compound,
for example, by generating an ion, a Lewis acid or a radical by
irradiation of an active energy ray such as visible light,
ultraviolet light, X-ray or electron beam. Examples of the
photoinitiator include onium salts such as aromatic diazonium
salts, aromatic iodonium salts or aromatic sulfonium salts,
acetophenone compounds, benzoin compounds, benzophenone compounds,
thioxanthone compounds and the like.
The window 300 may have a thickness of 50 to 100 .mu.m.
According to one embodiment of the present invention, a first
adhesive layer 50 may be formed between the display panel 100 and
the functional structure 200. In one embodiment, the first adhesive
layer 50 may make a direct contact with the surfaces of the display
panel 100 and the functional structure 200. For example, the first
adhesive layer 50 may be formed on the bottom surface of the
functional structure 200 (for example, the touch sensor, the
polarizing layer or the retardation layer), and then the exposed
surface of the first adhesive layer 50 may be adhered to the upper
surface of the display panel 100.
A second adhesive layer 90 may be formed between the functional
structure 200 and the window 300. In one embodiment, the second
adhesive layer 90 may make a direct contact with the surfaces of
the functional structure 200 and the window 300. For example, the
second adhesive layer 90 may be formed on the bottom surface of the
window 300, and then the exposed surface of the second adhesive
layer 90 may be adhered to the upper surface of the functional
structure 200.
As used herein, the term "adhesive layer" refers to including a
layer formed by an adhesive, as well as a layer formed by a
tackifier. The adhesive layer may be formed using a pressure
sensitive adhesive (PSA) composition or an optically clear adhesive
(OCA) composition.
The first and second adhesive layers 50, 90 may have a viscoelastic
property applicable to a flexible display while having an
appropriate adhesive strength so as not to cause peeling, bubbles,
or the like when bending occurs on the flexible display device.
In the flexible display device according to one embodiment of the
present invention, the first and second adhesive layers 50, 90 have
a tan .delta. of 0.8 to 1 at -20.degree. C.
The value of tangent delta (Tan .delta.) refers to a ratio of loss
elastic modulus (G'')/storage elastic modulus (G') of the adhesive
layers and is an index indicating easy deformation of the adhesive
layers. In the present invention, the method for measuring the
value of tangent delta of the adhesive layers is not particularly
limited, and for example, it can be measured by the method
described in Experimental Examples described below.
In one embodiment of the present invention, the value of tangent
delta of the first and second adhesive layers 50, 90 is controlled
to be 0.8 to 1 at a low temperature of -20.degree. C., thereby
improving the bending resistance and durability of the flexible
display device comprising the first and second adhesive layers
50,90.
In addition, the first and second adhesive layers 50, 90 may each
have a storage elastic modulus (G') of 0.05 to 0.15 MPa at
-20.degree. C.
If the storage elastic modulus of the first and second adhesive
layers 50, 90 is less than 0.05 MPa at -20.degree. C., the
durability may be deteriorated, and if the storage elastic modulus
is greater than 0.15 MPa, the bending resistance may be
deteriorated.
The thickness of the first and second adhesive layers 50, 90 may
each be 20 to 200 .mu.m, and preferably 20 to 100 .mu.m.
The storage elastic modulus at -20.degree. C. and thickness of the
first and second adhesive layers 50, 90 may be the same or
different.
The tan .delta. and the storage elastic modulus of the first and
second adhesive layers 50, 90 at -20.degree. C. may be controlled,
for example, by changing composition of adhesive compositions for
forming these layers.
The first and second adhesive layers 50, 90 may be formed using an
acrylic-based, a silicone-based, or a urethane-based adhesive
composition.
In one embodiment of the present invention, the first and second
adhesive layers 50, 90 may be formed from an adhesive composition
comprising a (meth)acrylate resin, a (meth)acrylate ester monomer
and a photoinitiator.
In particular, the first and second adhesive layers 50, 90 may be
formed from an adhesive composition comprising an aliphatic
(meth)acrylate polymer, a (meth)acrylate ester monomer and a
photoinitiator.
The aliphatic (meth)acrylate polymer is a component which serves to
control the viscosity-to-elasticity balance of the adhesive
composition, and for example, it may be an alkyl (meth)acrylate
monomer, in particular, a polymer of C.sub.4-C.sub.8 alkyl
(meth)acrylate monomers.
Specific examples of the alkyl (meth)acrylate monomer include
n-butyl (meth)acrylate, 2-butyl (meth)acrylate, t-butyl
(meth)acrylate, 2-ethylhexyl (meth)acrylate, ethyl (meth)acrylate,
methyl (meth)acrylate, n-propyl (meth)acrylate, isopropyl
(meth)acrylate, n-pentyl (meth)acrylate, n-octyl (meth)acrylate,
isooctyl (meth)acrylate and the like. Among them, a C.sub.4-C.sub.8
alkyl(meth)acrylate monomer such as n-butyl (meth)acrylate and
2-ethylhexyl (meth)acrylate may be preferably used. They may be
used alone or in a combination of two or more thereof.
The method of preparing the aliphatic (meth)acrylate polymer is not
particularly limited, and a bulk polymerization, an emulsion
polymerization, a suspension polymerization or the like which is
commonly used in the art may be used, and the bulk polymerization
is preferable. In addition, a polymerization initiator commonly
used during polymerization, a chain transfer agent for controlling
the molecular weight, or the like may be used.
The aliphatic (meth)acrylate polymer may be a partial polymer in
which the above-described monomers are partially polymerized.
Such a partial polymer may appear in a syrup form (viscous liquid
phase) in which the polymer formed from a part of the monomer and
the unreacted monomer coexist. Thus, the partial polymer having
such a property is sometimes referred to as a polymer syrup.
The method of partially polymerizing the monomer is not
particularly limited, and various polymerization methods may be
appropriately selected and used. For example, photopolymerization
or thermal polymerization methods may be used.
The solid content of the partial polymer, that is, the polymer
syrup, may be 20 to 60 wt %, and preferably 30 to 50 wt %. If the
solid content of the polymer syrup is less than 20 wt %, the
durability of the adhesive composition may be deteriorated, and if
the solid content is higher than 60 wt %, the coating property of
the adhesive composition may be deteriorated.
The solid content of the polymer syrup is equal to the amount of
polymer formed. For example, if the solid content is 40 wt %, the
amount of the polymer formed is 40 wt % based on 100 wt % of the
total weight of the polymer syrup.
In one embodiment of the present invention, the aliphatic
(meth)acrylate polymer may be contained in an amount of 30 to 50 wt
% based on 100 wt % of the total weight of the adhesive
composition. If the aliphatic (meth)acrylate polymer is contained
in an amount less than 30 wt %, the durability of the adhesive
composition may be deteriorated, and if it is contained in an
amount exceeding 50 wt %, the coating property of the adhesive
composition may be deteriorated.
The (meth)acrylate ester monomer is a component that can function
as a dispersion medium for the aliphatic (meth)acrylate polymer and
can be polymerized by the action of a photoinitiator.
The (meth)acrylate ester monomer may include an alkyl
(meth)acrylate used in the preparation of the aliphatic
(meth)acrylate polymer.
In addition, the (meth)acrylate ester monomer may further include a
C.sub.10-C.sub.20 alkyl (meth)acrylate.
Specific examples of the C.sub.10-C.sub.20 alkyl (meth)acrylate
include decyl (meth)acrylate, isodecyl (meth)acrylate, undecyl
(meth)acrylate, lauryl (meth)acrylate, tridecyl (meth)acrylate and
the like. They may be used alone or in combination of two or more
thereof.
In one embodiment of the present invention, the (meth)acrylate
ester monomer may be contained in an amount of 45 to 70 wt % based
on 100 wt % of the total weight of the adhesive composition. If the
content of the (meth)acrylate ester monomer satisfies the
above-mentioned range, the tan .delta. and the storage elastic
modulus at -20.degree. C. can be controlled within an appropriate
range.
The photoinitiator may be used without particular limitation as
long as it is an initiator being used in the art. The
photoinitiator is a Type I photoinitiator in which radicals are
generated by decomposition of molecules due to a difference in
chemical structure or molecular binding energy, and a Type II
(hydrogen abstraction type) photoinitiator in which tertiary amines
are incorporated as a co-initiator. Specific examples of the Type I
photoinitiator may include acetophenones such as
4-phenoxydichloroacetophenone, 4-t-butyldichloroacetophenone,
4-t-butyltrichloroacetophenone, diethoxyacetophenone,
2-hydroxy-2-methyl-1-phenylpropan-1-one,
1-(4-isopropylphenyl)-2-hydroxy-2-methylpropan-1-one,
1-(4-dodecylphenyl)-2-hydroxy-2-methylpropan-1-one,
4-(2-hydroxyethoxy)-phenyl(2-hydroxy-2-propyl)ketone,
1-hydroxycyclohexyl phenyl ketone or the like, benzoins such as
benzoin, benzoin methyl ether, benzoin ethyl ether, benzyl dimethyl
ketal or the like, acylphosphine oxides, and titanocene compounds
Specific examples of the Type II photoinitiator may include
benzophenones such as benzophenone, benzoyl benzoic acid. benzoyl
benzoic acid methyl ether, 4-phenylbenzophenone,
hydroxybenzophenone, 4-benzoyl-4'-methyldiphenylsulfide,
3,3'-methyl-4-methoxybenzophenone or the like, and thioxanthones
such as thioxanthone, 2-chlorothioxanthone, 2-methylthioxanthone,
2,4-dimethylthioxanthone, isopropylthioxanthone or the like. These
photoinitiators may be used alone or in combination of two or more.
In addition, Type I photoinitiator and Type II photoinitiator may
be used alone or in combination.
The photoinitiator may be contained in an amount of 0.01 to 5.0 wt
% based on 100 wt % of the total weight of the adhesive
composition. If the photoinitiator is contained in an amount less
than 0.01 wt %, it may be difficult to effectively initiate the
cross-linking reaction, and if it is contained in an amount
exceeding 5.0 wt %, discoloration or the like may occur due to
residual initiator, thereby causing a deterioration of
durability.
In one embodiment of the present invention, the first and second
adhesive layers 50, 90 may be formed from an adhesive composition
comprising a silicone urethane (meth)acrylate resin, a
(meth)acrylate ester monomer and a photoinitiator.
The silicone urethane (meth)acrylate resin may be a resin having a
silicon bond (--Si--O--) and a urethane group in the molecule and
having a (meth)acryloyloyloxy terminal group.
The silicone urethane (meth)acrylate resin may be a reaction
product of a polysilicon having hydroxyl groups at both ends, a
polyisocyanate, and a (meth)acrylate having a hydroxyl group.
The polysilicon having hydroxyl groups at both ends may be a
compound represented by Chemical Formula 1.
##STR00001##
wherein.
R.sup.1 and R.sup.3 are each independently a C.sub.1-C.sub.10
alkylene group or a C.sub.1-C.sub.10 oxyalkylene group,
R.sup.2 is a C.sub.1-C.sub.10 alkyl group, a C.sub.3-C.sub.10
cycloalkyl group or an aryl group, and n is an integer of 1 to
30.
As used herein, the C.sub.1-C.sub.10 alkylene group refers to a
linear or branched divalent hydrocarbon having 1 to 10 carbon
atoms, and examples thereof include methylene, ethylene, propylene,
butylene, pentylene and the like, but are not limited thereto.
As used herein, the C.sub.1-C.sub.10 oxyalkylene group refers to a
functional group in which at least one carbon chain in a linear or
branched divalent hydrocarbon having 1 to 10 carbon atoms is
substituted with oxygen, and examples thereof include
ethyloxypropyl, propyloxyethyl, and the like, but are not limited
thereto.
As used herein, the C.sub.1-C.sub.10 alkyl group refers to a linear
or branched monovalent hydrocarbon having 1 to 10 carbon atoms, and
examples thereof include methyl, ethyl, n-propyl, i-propyl,
n-butyl, i-butyl, t-butyl, n-pentyl, n-hexyl, and the like, but are
not limited thereto.
As used herein, the C.sub.3-C.sub.10 cycloalkyl group refers to a
simple or fused ring hydrocarbon having 3 to 10 carbon atoms, and
examples thereof include cyclopropyl, cyclobutyl, cyclopentyl,
cyclohexyl and the like, but are not limited thereto.
As used herein, the aryl group includes all of aromatic groups,
heteroaromatic groups and partially-reduced derivatives thereof.
The aromatic group refers to a 5 to 15-membered simple or fused
ring, and the heteroaromatic group refers to an aromatic group
containing at least one of oxygen, sulfur or nitrogen. Typical
examples of the aryl group include phenyl, naphthyl, pyridinyl,
furanyl, thiophenyl, indolyl, quinolinyl, imidazolinyl, oxazolyl,
thiazolyl, tetrahydronaphthyl, and the like, but are not limited
thereto.
The weight average molecular weight of the polysilicon having
hydroxyl groups at both ends may be 800 to 2,000.
As the polyisocyanate, an aliphatic diisocyanate, an alicyclic
diisocyanate, and/or an aromatic diisocyanate may be used.
Examples of the aliphatic diisocyanate include methyl diisocyanate,
1,2-ethanediyl diisocyanate, 1,3-propanediyl diisocyanate,
1,6-hexanediyl diisocyanate, 3-methyl-octane-1,8-diyl diisocyanate
and the like.
Examples of the alicyclic diisocyanate include
4,4'-methylenedicyclohexyl diisocyanate, 1,2-cyclopropanediyl
diisocyanate, 1,3-cyclobutanediyl diisocyanate, 1,4-cyclohexanediyl
diisocyanate, 1,3-cyclohexanediyl diisocyanate, isophorone
diisocyanate, 4-methyl-cyclohexane-1,3-diyl diisocyanate, and the
like.
Examples of the aromatic diisocyanate include 1,2-phenylene
diisocyanate, 1,3-phenylene diisocyanate, 1,4-phenylene
diisocyanate, 3-chloro-1,2-benzene diisocyanate,
4-chloro-1,2-benzene diisocyanate, 5-chloro-1,2-benzene
diisocyanate, 2-chloro-1,3-benzene diisocyanate,
4-chloro-1,3-benzene diisocyanate, 5-chloro-1,3-benzene
diisocyanate, 2-chloro-1,4-benzene diisocyanate,
3-chloro-1,4-benzene diisocyanate, 3-methyl-1,2-benzene
diisocyanate, 4-methyl-1,2-benzene diisocyanate,
5-methyl-1,2-benzene diisocyanate, 2-methyl-1,3-benzene
diisocyanate, 4-methyl-1,3-benzene diisocyanate,
5-methyl-1,3-benzene diisocyanate, 2-methyl-1,4-benzene
diisocyanate, 3-methyl-1,4-benzene diisocyanate,
3-methoxy-1,2-benzene diisocyanate, 4-methoxy-1,2-benzene
diisocyanate, 5-methoxy-1,2-benzene diisocyanate,
2-methoxy-1,3-benzene diisocyanate, 4-methoxy-1,3-benzene
diisocyanate, 5-methoxy-1,3-benzene diisocyanate,
2-methoxy-1,4-benzene diisocyanate, 3-methoxy-1,4-benzene
diisocyanate, 3,4-dimethyl-1,2-benzene diisocyanate,
4,5-dimethyl-1,3-benzene diisocyanate, 2,3-dimethyl-1,4-benzene
diisocyanate, 3-chloro-4-methyl-1,2-benzene diisocyanate,
3-methyl-4-chloro-1,2-benzene diisocyanate,
3-methyl-5-chloro-1,2-benzene diisocyanate,
2-chloro-4-methyl-1,3-benzene diisocyanate,
4-chloro-5-methoxy-1,3-benzene diisocyanate,
5-chloro-2-fluoro-1,3-benzene diisocyanate,
2-chloro-3-bromo-1,4-benzene diisocyanate,
3-chloro-5-isopropoxy-1,4-benzene diisocyanate, 2,3-diisocyanate
pyridine, 2,4-diisocyanate pyridine, 2,5-diisocyanate pyridine,
2,6-diisocyanate pyridine, 2,5-diisocyanate-3-methylpyridine,
2,5-diisocyanate-4-methylpyridine,
2,5-diisocyanate-6-methylpyridine and the like.
Examples of the (meth)acrylate having a hydroxyl group include
(meth)acrylic acid alkyl esters having a hydroxyl group such as
2-hydroxyethyl (meth)acrylate, 2-hydroxypropyl (meth)acrylate,
3-hydroxypropyl (meth)acrylate, 3-hydroxybutyl (meth)acrylate,
4-hydroxybutyl (meth)acrylate, 6-hydroxyhexyl (meth)acrylate and
the like; polyfunctional (meth)acrylates having a hydroxyl group
such as trimethylolpropane di(meth)acrylate, pentaerythritol
tri(meth)acrylate, dipentaerythritol penta(meth)acrylate and the
like; polyethylene glycol monoacrylate, polypropylene glycol
monoacrylate, and the like.
The silicone urethane (meth)acrylate resin may be prepared, for
example, by introducing the polysilicon having hydroxyl groups at
both ends and the (meth)acrylate having a hydroxyl group into the
reaction system, supplying the polyisocyanate, mixing them, and
reacting the mixture under a solvent free condition, or
alternatively reacting the polysilicon having hydroxyl groups at
both ends and the polyisocyanate to obtain a urethan prepolymer
having an isocyanate group, supplying the (meth)acrylate having a
hydroxyl group, mixing them, and reacting the mixture under a
solvent free condition. The reaction may be carried out at 20 to
120.degree. C. for 30 minutes to 24 hours.
In the preparation of the silicone urethane (meth)acrylate resin, a
polymerization inhibitor, an urethanation catalyst and the like may
be used, if necessary.
The weight average molecular weight (hereinafter, referred to as
"weight average molecular weight") in terms of polystyrene measured
by a gel permeation chromatography (GPC, tetrahydrofuran as an
elution solvent) of the silicone urethane (meth)acrylate resin is
not particularly limited, and for example, it may be 1,500 to
3,000
In one embodiment of the present invention, the silicone urethane
(meth)acrylate resin may be contained in an amount of 30 to 50 wt %
based on 100 wt % of the total weight of the adhesive composition.
If the silicone urethane (meth)acrylate resin is contained in an
amount less than 30 wt %, the durability of the adhesive
composition may be deteriorated, and if it is contained in an
amount exceeding 50 wt %, the coating property of the adhesive
composition may be deteriorated.
As the (meth)acrylate ester monomer, an aliphatic (meth)acrylate, a
(meth)acrylate having an ether group, a (meth)acrylate having a
hydroxyl group, a (meth)acrylamide having a nitrogen atom, and the
like may be used, and in particular, an aliphatic (meth)acrylate
may be used.
Examples of the aliphatic (meth)acrylate include methyl
(meth)acrylate, ethyl (meth)acrylate, propyl (meth)acrylate,
isopropyl (meth)acylate, butyl (meth)acrylate, sec-butyl
(meth)acrylate, isobutyl (meth)acrylate, 2-ethylbutyl
(meth)acrylate, n-pentyl (meth)acrylate, hexyl (meth)acrylate,
2-ethylhexyl (meth)acrylate, heptyl (meth)acrylate, n-octyl
(meth)acrylate, nonyl (meth)acrylate, decyl (meth)acrylate,
isodecyl (meth)acrylate, dodecyl (meth)acrylate, 3-methylbutyl
(meth)acrylate, isooctyl (meth)acrylate, lauryl (meth)acrylate,
tridecyl (meth)acrylate, stearyl (meth)acrylate, isostearyl
(meth)acrylate, neopentyl (meth)acrylate, hexadecyl (meth)acrylate,
isoamyl (meth)acrylate and the like. In particular, a
C.sub.10-C.sub.20 alkyl (meth)acrylate such as lauryl
(meth)acrylate, isodecyl (meth)acrylate and the like may be
preferably used.
Examples of the (meth)acrylate having an ether group include
3-methoxybutyl (meth)acrylate, 2-methoxyethyl (meth)acrylate,
3-methoxypropyl (meth)acrylate, 2-methoxybutyl (meth)acrylate,
methoxy polyethylene glycol acrylate having an addition mole number
of oxyethylene ranging from 1 to 15, ethoxy-diethylene glycol
(meth)acrylate, ethyl carbitol (meth)acrylate and the like.
Examples of the (meth)acrylate having a hydroxyl group include
2-hydroxyethyl (meth)acrylate, 2-hydroxypropyl (meth)acrylate,
4-hydroxybutyl (meth)acrylate and the like.
Examples of the (meth)acrylamide having a nitrogen atom include
(meth)acrylamide, dimethyl (meth)acrylamide, acryloylmorpholine,
dimethylaminopropyl (meth)acrylamide, isopropyl (meth)acrylamide,
diethyl (meth)acrylamide, hydroxyethyl (meth)acrylamide, diacetone
(meth)acrylamide and the like.
In one embodiment of the present invention, the (meth)acrylate
ester monomer may be contained in an amount of 45 to 70 wt % based
on 100 wt % of the total weight of the adhesive composition. If the
content of the (meth)acrylate ester monomer satisfies the above
range, the tan .delta. and storage elastic modulus at -20.degree.
C. may be controlled within an appropriate range.
The photoinitiator refers to an initiator that absorbs an active
energy ray to generate a radical. Specific examples of the
photoinitiator include benzoin compounds, benzophenone compounds,
thioxanthone compounds, acetophenone compounds, and the like.
The photoinitiator may be contained in an amount of 0.01 to 5.0 wt
% based on 100 wt % of the total weight of the adhesive
composition. If the photoinitiator is contained in an amount less
than 0.01 wt %, it may be difficult to effectively initiate the
crosslinking reaction. If the photoinitiator is contained in an
amount exceeding 5.0 wt %, discoloration or the like may occur due
to residual initiator, thereby causing a deterioration of
durability.
According to one embodiment of the present invention, as the first
and second adhesive layers 50, 90 are inserted into respective
layers or respective elements included in the flexible display
device, a strain separation may occur when a stress such as tension
or compression is generated in the flexible display device.
Thus, a neutral plane (NP) may be formed in respective layers or
respective elements included in the flexible display device.
As illustrated in FIG. 1, a first neutral plane (NP1) may be formed
in a display panel 100, a second neutral plane (NP2) may be formed
in a functional structure 200, and a third neutral plane (NP3) may
be formed in a window 300.
The tan .delta. of the first and second adhesive layers 50, 90 at
-20.degree. C. may be adjusted so that the neutral planes are
formed by the strain separation through the first and second
adhesive layers 50, 90 at a low temperature of -20.degree. C. As
described above, the neutral planes are formed in the respective
elements of the flexible display device by the first and second
adhesive layers 50, 90 at a low temperature of -20.degree. C. and
therefore, the bending property may be improved at a low
temperature of -20.degree. C., and damage caused by stress in the
respective layers of the display panel 100, the functional
structure 200, and the window 300 may be suppressed.
The formation of the neutral planes through the first and second
adhesive layers 50, 90 will be described below in detail with
reference to FIG. 4.
FIG. 2 is a cross-sectional view schematically illustrating the
display panel of the flexible display device according to one
embodiment of the present invention.
Referring to FIG. 2, the display panel 100 may include a thin film
transistor (TFT), an insulating structure, a pixel electrode 145,
and a display layer 160 disposed on a panel substrate 105.
The panel substrate 105 may be provided as a base substrate of the
display panel 100. The panel substrate 105 may include, for
example, a flexible resin material such as polyimide.
The thin film transistor may include an active layer 110, a gate
electrode 120, a source electrode 132, and a drain electrode 134.
The insulating structure may include a gate insulating film 115, an
interlayer insulating film 125, and a via-insulating film 140.
The active layer 110 may be formed on the upper surface of the
panel substrate 105 and may include polysilicon or an oxide
semiconductor (for example, indium gallium zinc oxide (IGZO) or the
like). The gate insulating film 115 may be formed on the panel
substrate 105 and may cover the active layer 110.
The gate electrode 120 may be disposed on the gate insulating film
115 and may overlay the active layer 110. The gate electrode 120
may include a metal such as Al, Ti, Cu, W, Ta, or Ag.
After an interlayer insulating film 125 covering the gate electrode
120 is formed on the gate insulating film 115, a source electrode
132 and a drain electrode 134 which make a contact with the active
layer 110 may be formed by penetrating the interlayer insulating
film 125 and the gate insulating film 115. The source electrode 132
and the drain electrode 134 may include a metal such as Al, Ti, Cu,
W, Ta, or Ag.
A via insulating film 140 which overlays the source electrode 132
and the drain electrode 134 may be formed on the interlayer
insulating film 125. The via insulating film 140 may be formed
using an organic insulating material such as an acrylic-based
resin, a siloxane-based resin, or the like
A pixel electrode 145 that is electrically connected to the drain
electrode 134 may be formed on the via insulating film 140. The
pixel electrode 145 may include a via portion making a contact with
the drain electrode 134 by penetrating the via insulating film 140.
The pixel electrode 145 may include a metal such as Al, Ti, Cu, W,
Ta or Ag, and/or a transparent conductive oxide (for example,
indium tin oxide (ITO)).
A pixel defining film 150 may be formed on the via insulating film
140, and a display layer 160 may be formed on the upper surface of
the pixel electrode 145 exposed by the pixel defining film 150. The
display layer 160 may be formed, for example, as an organic
emissive layer (EML) contained in an OLED device or a liquid
crystal layer contained in an LCD device.
A counter electrode 170 may be formed on the pixel defining film
150 and the display layer 160. The counter electrode 170 may be
provided as a common electrode, a reflective electrode, or a
cathode of an image display device.
The display panel 100 includes a plurality of pixels, and the thin
film transistor and the pixel electrode 145 may be arranged for
each pixel. The counter electrode 170 may be provided as a common
electrode that continuously extends on a plurality of pixels.
An encapsulation layer 180 may be formed on the counter electrode
170. The encapsulation layer 180 may include an inorganic
insulating material such as silicon oxide, silicon nitride, and/or
an organic insulating material such as acryl-based, siloxane-based,
urethane-based resin or the like.
The display panel 100 may further include a pixel circuit which is
electrically connected to the thin film transistor. For example,
the pixel circuit may include a data line, a scan line, a power
supply line, and the like, and each pixel may be independently
defined for every intersection of the data line and the scan
line.
As described above, the first neutral plane (NP1) is formed inside
the display panel 100 at a low temperature of -20.degree. C., and
thus, damage such as cracks, breakage or the like of the pixel
circuit, the thin film transistor, and the electrode may be
suppressed when the display panel is bent at a low temperature of
-20.degree. C.
The thickness of the display panel 100 may be 80 to 400 .mu.m.
FIG. 3 is a plan view schematically illustrating a touch sensor of
the flexible display device according to one embodiment of the
present invention. For example, the touch sensor may be applied to
the functional structure 200 in the form of a touch screen panel or
a touch sensor film.
Referring to FIG. 3, the touch sensor may include a sensing
electrode 210, a trace 220, and a pad 230 formed on a substrate
205.
The touch sensor may include a sensing area (A) and a peripheral
area (B). The sensing electrode 210 may be formed on the substrate
205 of the sensing area (A), and the trace 220 and the pad 230 may
be formed on the substrate 205 of the peripheral area (B).
The sensing electrode 210 may include, for example, a first sensing
electrode 210a and a second sensing electrode 210b arranged along a
first direction and a second direction that are parallel to the
upper surface of the substrate 205 and intersect perpendicularly to
each other.
The first sensing electrode 210a may extend in the first direction,
and a plurality of first sensing electrodes 210a may be formed
along the second direction. The second sensing electrode 210b may
extend in the second direction, and a plurality of second sensing
electrodes 210b may be formed along the first direction.
Each of the first and second sensing electrodes 210a, 210b may
include, for example, a unit pattern in polygonal shape, and may
include a connection unit or a bridge electrode that connects
neighboring unit patterns. The inside of the unit pattern may
include a conductive pattern patterned in a mesh type.
The trace 220 may be branched from the respective sensing
electrodes 210a, 210b, and the terminal of the trace 220 may be
connected to a pad 230. The touch sensor may be connected, for
example, to an external circuit such as a flexible printed circuit
board (FPCB) through the pad 230.
As described above, the second neutral plane (NP2) is formed inside
the touch sensor at a low temperature of -20.degree. C. Thus,
damage such as breakage, cracks or the like of the sensing
electrode 210, the bridge electrode, and the trace 220 may be
prevented when it is bent at a low temperature of -20.degree.
C.
FIG. 4 is a graph illustrating a strain profile of the flexible
display device according to one embodiment of the present
invention.
Referring to FIG. 4, when the flexible display device is bent at a
low temperature of -20.degree. C. the neutral plane may be formed
in each of the display panel 100, the functional structure 200, and
the window 300, as described with reference to FIG. 1.
As used herein, the term "neutral plane" is defined as a surface in
which a compressive stress (denoted by "-" in FIG. 4) and a tensile
stress (denoted by "+" in FIG. 4) are substantially the same when
the flexible display device is bent. Accordingly, the strain can be
converged substantially to zero on the neutral plane.
When the flexible display device is bent, a compressive force may
be applied on the inner side on which it is bent, and a tensile
force is applied on the outer side, and thus, damage may occur in
the internal structure thereof.
According to one embodiment of the present invention, when the
flexible display device is bent at a low temperature of -20.degree.
C., the optical elements or the information transfer elements
included in the flexible display device all include the neutral
plane, and therefore, the compressive force and the tensile force
are internally offset, so that breakage, crack and damage of the
internal structure can be prevented. Thus, the range within which
the flexible display device can be curved or bent at a low
temperature of -20.degree. C. may be extended, thereby improving
both flexibility and mechanical reliability.
As illustrated in FIG. 4, a strain separation may be generated by
the first adhesive layer 50 between the display panel 100 and the
functional structure 200, and thus, the first neutral plane (NP1)
and the second neutral plane (NP2) may be formed in the display
panel 100 and the functional structure 200, respectively.
In addition, a strain separation may be generated by the second
adhesive layer 90 between the functional structure 200 and the
window 300, and thus, the third neutral plane (NP3) may be formed
in the window 300.
In one embodiment of the present invention, the total thickness of
the flexible display device may range from 200 to 1,000 .mu.m.
FIG. 5 is a cross-sectional view schematically illustrating the
flexible display device according to another embodiment of the
present invention.
Referring to FIG. 5, the functional structure may comprise a touch
sensor 200a and a polarizing layer 200b. Further, a third adhesive
layer 70 may be formed between the touch sensor 200a and the
polarizing layer 200b.
The third adhesive layer 70 may have a tan .delta. of 0.8 to 1 at
-20.degree. C. Accordingly, the bending resistance and durability
of the flexible display device comprising the third adhesive layer
70 may be improved.
In addition, the storage elastic modulus of the third adhesive
layer 70 at -20.degree. C. may be 0.05 to 0.15 MPa. If the storage
elastic modulus of the third adhesive layer 70 at -20.degree. C. is
less than 0.05 MPa, the durability may be deteriorated. If the
storage elastic modulus is higher than 0.15 MPa, the bending
resistance may be deteriorated.
The thickness of the third adhesive layer 70 may be 5 to 30
.mu.m.
An adhesive composition for forming the third adhesive layer 70 may
be the same as the adhesive compositions for forming the first and
second adhesive layers 50, 90.
When the flexible display device is bent at a low temperature of
-20.degree. C., a strain separation between the touch sensor 200a
and the polarizing layer 200b may be promoted by the third adhesive
layer 70, and thus, a neutral plane may be formed inside of each of
the touch sensor 200a and the polarizing layer 200b.
For example, a second neutral plane (NP2) may be formed inside the
touch sensor 200a, and a third neutral plane (NP3) may be formed
inside the polarizing layer 200b. A fourth neutral plane (NP4) may
be formed inside the window 300 by the strain separation through
the second adhesive layer 90 as described above.
FIG. 6 is a cross-sectional view schematically illustrating the
flexible display device according to still another embodiment of
the present invention.
Referring to FIG. 6, the flexible display device may further
comprise, for example, a protective film 350 exposed to the user
viewing side, as an outermost layer. For example, the protective
film 350 may be laminated on the window 300.
In one embodiment of the present invention, the protective film 350
may be adhered on the upper surface of the window 300 through a
fourth adhesive layer 80.
The fourth adhesive layer 80 may have a tan .delta. of 0.8 to 1 at
-20.degree. C., as in the first and second adhesive layers 50, 90
described above. Accordingly, the bending resistance and durability
of the flexible display device comprising the fourth adhesive layer
80 may be improved.
In addition, the storage elastic modulus of the fourth adhesive
layer 80 may be 0.05 to 0.15 MPa at -20.degree. C. If the storage
elastic modulus of the fourth adhesive layer 80 is less than 0.05
MPa at -20.degree. C., the durability may be deteriorated. If the
storage elastic modulus is higher than 0.15 MPa, the bending
resistance may be deteriorated.
The thickness of the fourth adhesive layer 80 may be 5 to 30
.mu.m.
An adhesive composition for forming the fourth adhesive layer 80
may be the same as the adhesive compositions for forming the first
and second adhesive layers 50, 90.
The protective film 350 may include a flexible resin film, and for
example, it may include a polyimide film. The protective film 350
may have a thickness of 20 to 50 .mu.m.
When the flexible display device is bent at a low temperature of
-20.degree. C., a neutral plane (for example, the fourth neutral
plane (NP4)) may be formed inside the protective film 350 by the
fourth adhesive layer 80. Therefore, the durability and crack
resistance of the protective film 350 against an external impact
can be further improved, thereby protecting the flexible display
device.
Hereinafter, the present invention will be described in more detail
by way of Examples, Comparative Examples and Experimental Examples.
However, these Examples, Comparative Examples and Experimental
Examples are given for illustrating the present invention only, and
it will be apparent to those skilled in the art that the scope of
the invention is not intended to be limited thereto.
Preparation Example 1: Preparation of Silicone Urethan
(meth)acrylate Resin
To a 2 L three-necked flask equipped with an electric stirrer, a
heating mantle, a cooling tube and a temperature controller.
Polyisocyanate, polysilicon having hydroxyl groups at both ends,
and dibutyltin dilaurate as a catalyst were added in the
composition (unit: parts by weight) shown in Table 1 below, the
reaction temperature was raised to 70.degree. C. while stirring,
and the reaction was continued for 4 hours. Then, (meth)acrylate
having a hydroxyl group and methoxyhydroquinone as a polymerization
inhibitor were added dropwise in the composition shown in Table 1
below and allowed to react. After completion of the addition, the
reaction was continued for 2 hours. When the peak at 2260
cm.sup.-1, which is a characteristic peak of NCO on the FT-IR,
disappears, the reaction was terminated to obtain a silicone
urethane (meth)acrylate resin.
TABLE-US-00001 TABLE 1 Preparation Example 1 Polyisocyanate .sup.1)
524 Polysilicon having hydroxyl groups at both 933 ends .sup.2)
Dibutyltin dilaurate 0.4 Methoxyhydroquinone 0.3 (Meth)acrylate
having a hydroxyl group .sup.3) 232 .sup.1) 4,4'-methylene
dicyclohexyl diisocyanate (molecular weight of 262) .sup.2)
X-22-160AS (Shin-Etsu, molecular weight of 933) .sup.3)
2-hydroxyethyl acrylate (molecular weight of 116)
Preparation Example 2: Preparation of (meth)acrylate Syrup
To a 1 L reactor with nitrogen gas refluxed and an installed
refrigerator to easily regulate a temperature, 2-ethylhexyl
acrylate (2-EHA) monomer was added, and nitrogen gas was purged for
1 hour to remove oxygen, and then the temperature was maintained at
80.degree. C. 0.5 parts by weight of 1-hydroxycyclohexyl phenyl
ketone as a photoinitiator was added based on 100 parts by weight
of the monomer. Subsequently, the reaction mixture was irradiated
with a UV lamp (10 mW) while stirring to prepare a (meth)acrylate
syrup having a solid content of 40 wt %, that is, a mixture of 40
parts by weight of (meth)acrylate polymer and 60 parts by weight of
2-ethylhexyl acrylate (2-EHA) monomer. It was then used in the form
of the (meth)acrylate syrup in a subsequent preparation of the
adhesive composition.
Preparation Example 3: Preparation of (meth)acrylate Syrup
To a 1 L reactor with nitrogen gas refluxed and an installed
refrigerator to easily regulate a temperature, n-butyl acrylate
(BA) monomer was added, and nitrogen gas was purged for 1 hour to
remove oxygen, and then the temperature was maintained at
80.degree. C. 0.5 parts by weight of 1-hydroxycyclohexyl phenyl
ketone as a photoinitiator was added based on 100 parts by weight
of the monomer. Subsequently, the reaction mixture was irradiated
with a UV lamp (10 mW) while stirring to prepare a (meth)acrylate
syrup having a solid content of 40 wt %, that is, a mixture of 40
parts by weight of (meth)acrylate polymer and 60 parts by weight of
n-butyl acrylate (BA) monomer. It was then used in the form of the
(meth)acrylate syrup in a subsequent preparation of the adhesive
composition.
Preparation Example 4: Preparation of (meth)acrylate Syrup
To a 1 L reactor with nitrogen gas refluxed and an installed
refrigerator to easily regulate a temperature, tetrahydrofurfuryl
acrylate (THFA) monomer was added, and nitrogen gas was purged for
1 hour to remove oxygen, and then the temperature was maintained at
80.degree. C. 0.5 parts by weight of 1-hydroxycyclohexyl phenyl
ketone as a photoinitiator was added based on 100 parts by weight
of the monomer. Subsequently, the reaction mixture was irradiated
with a UV lamp (10 mW) while stirring to prepare a (meth)acrylate
syrup having a solid content of 40 wt %, that is, a mixture of 40
parts by weight of (meth)acrylate polymer and 60 parts by weight of
tetrahydrofurfuryl acrylate (THFA) monomer. It was then used in the
form of the (meth)acrylate syrup in a subsequent preparation of the
adhesive composition.
Examples 1 to 5 and Comparative Examples 1 to 4: Preparation of
Flexible Display Device
The adhesive compositions were prepared by mixing the respective
components in the composition shown in Table 2 below (unit: wt
%)
TABLE-US-00002 TABLE 2 Exam- Exam- Comparative Comparative
Comparative Comparative ple 1 ple 2 Example 3 Example 4 Example 5
Example 1 Example 2 Example 3 Example 4 Silicone urethane 40 50 30
-- -- 70 75 5 -- (meth)acrylate resin of Preparation Example 1
(Meth)acrylate polymer -- -- -- 28 16 -- -- -- -- of Preparation
Example 2 (Meth)acrylate polymer -- -- -- 8 16 -- -- -- -- of
Preparation Example 3 (Meth)acrylate polymer -- -- -- -- -- -- --
-- 38.8 of Preparation Example 4 Photoinitiator.sup.1) 3 3 3 3 3 3
3 3 3 Lauryl acrylate 30 25 50 7 -- -- 12 45 -- Isodecyl acrylate
27 22 17 -- 17 27 10 47 -- 2-Ethylhexyl acrylate -- -- -- 42 24 --
-- -- -- n-Butyl acrylate -- -- -- 12 24 -- -- -- --
Tetrahydrofurfuryl -- -- -- -- -- -- -- -- 58.2 acrylate
.sup.1)1-hydroxycyclohexyl phenyl ketone
The respective adhesive compositions prepared above were coated
onto a releasing film coated with a silicone releasing agent so
that the thickness thereof is 25 .mu.m, and a releasing film was
adhered thereto, and then the resultant was irradiated at a light
intensity of 500 mJ/cm.sup.2 using a high pressure mercury UV lamp
to prepare respective adhesive sheets.
A pixel electrode, an organic emissive layer (display layer), a
counter electrode, and an encapsulation layer were sequentially
formed on a panel substrate (polyimide, thickness of 30 .mu.m)
having a width and a length of 10 cm each, thereby preparing a
display panel having a total thickness of 120 .mu.m.
The respective adhesive sheets prepared above were laminated on the
display panel to form a first adhesive layer.
Next, a touch sensor (including an ITO electrode pattern layer
formed on a TAC film) and a polarizing plate including a stretched
polarizer (which is a PVA polarizer film obtained by adhering a TAC
protective film on both surfaces thereof and a retardation film was
attached to one surface thereof) were laminated on the first
adhesive layer to form a functional structure. The thickness of the
total functional structure was 100 rm.
The respective adhesive sheets prepared above were laminated on the
functional structure to form a second adhesive layer.
A window substrate (polyimide, thickness of 70 .mu.m) was further
laminated on the second adhesive layer to prepare a flexible
display device.
Experimental Example 1: Evaluation of Physical Properties of
Flexible Display Device
The physical properties of the flexible display devices prepared in
Examples and Comparative Examples were measured by the following
methods, and the results are shown in Table 3 below.
(1) Tangent Delta and Storage Elastic Modulus (G')
Tangent delta and storage elastic modulus (G') were measured at
-20.degree. C. using a viscoelasticity measuring device (MCR-301,
manufactured by Anton Paar). Specifically, the sample size of the
adhesive sheets prepared in Examples and Comparative Examples was
made into a size of 30 mm length.times.30 mm wide. The measurement
sample was adhered to a glass plate, and then the tangent delta and
storage elastic modulus were measured under a condition with a
frequency of 1.0 Hz and deformation of 2% while it was adhered to a
measuring tip. The tangent delta is a value calculated by G''(loss
elastic modulus)/G'(storage elastic modulus).
(2) Bending Resistance
The bending resistance was evaluated using a folding tester
(DLDMLH-FS, manufactured by YUASA). The flexible display devices
prepared in Examples and Comparative Examples were cut into a size
of 100 mm.times.10 mm to prepare samples. When the samples were
mounted on the tester, each sample was reversely adhered so that a
bending tensile was applied to the organic light emitting layer of
one sample and a bending compressive stress was applied to the
organic light emitting layer of another sample. The samples were
mounted on the tester, and then 200,000 cycles of evaluation were
conducted under a condition of 3R and a rate of 30 cycles/min.
Then, the bending part of the sample was observed by an optical
microscope (ECLIPSE LV100ND, manufactured by NIKON), and the
bending resistance was evaluated based on the following
criteria.
<Evaluation Criteria>
.circleincircle.: No crack
.smallcircle.: the number of cracks is equal to or less than 5, and
it does not grow to the full length in the width direction
.DELTA.: the number of cracks is more than 5 and not more than 10,
and it does not grow to the full length in the width direction
x: the number of cracks is equal to or more than 11, and cracks
exist in the full length of the width direction
(3) Mandrel
In order to evaluate the bending properties, the flexible display
devices prepared in Examples and Comparative Examples were cut into
a size of 1 cm.times.10 cm to prepare samples. The samples were
each placed on steel rods having each diameter of 2.phi. to
20.phi., and the window substrate was directed downward and was
folded by hand, and the smallest diameter at which no crack or
streak appears on the surface was indicated.
(4) Heat Resistance Durability
The heat resistance durability was evaluated based on the following
criteria by allowing the flexible display devices prepared in
Examples and Comparative Examples to stand at a temperature of
100.degree. C. for 500 hours and then observing whether bubbles or
peeling occurred.
<Evaluation Criteria>
.circleincircle.: No bubble or peeling
.smallcircle.: less than 5 bubbles or peeling
.DELTA.: equal to or more than 5 and less than 10 bubbles or
peeling
x: equal to or more than 10 bubbles or peeling
TABLE-US-00003 TABLE 3 Heat Bending resistance Tan .delta. G'(MPa)
resistance Mandrel durability Example 1 0.9 0.1 .circleincircle.
3.phi. .circleincircle. Example 2 0.8 0.14 .circleincircle. 4.phi.
.circleincircle. Example 3 1 0.07 .circleincircle. 2.phi.
.circleincircle. Example 4 0.9 0.09 .circleincircle. 3.phi.
.circleincircle. Example 5 0.8 0.13 .circleincircle. 3.phi.
.circleincircle. Comparative 0.6 0.7 X 11.phi. X Example 1
Comparative 0.4 0.8 X 14.phi. X Example 2 Comparative 2 0.01 X
10.phi. X Example 3 Comparative 0.3 1.1 X 20.phi. X Example 4
As shown in Table 3, it was confirmed that the flexible display
devices of Examples 1 to 5 including the first adhesive layer and
the second adhesive layer having a tan .delta. of 0.8 to 1 at
-20.degree. C. had excellent bending resistance and durability. In
contrast, it was also confirmed that the flexible display devices
of Comparative Examples 1 to 4 including the first adhesive layer
and the second adhesive layer in which the tan .delta. at
-20.degree. C. is out of the range of 0.8 to 1 had poor bending
resistance and durability.
Although specific parts of the present invention have been
described in detail, it will be apparent to those skilled in the
art that these specific techniques are merely a preferred
embodiment and that the scope of the present invention is not
limited thereto. In addition, those skilled in the art will
appreciate that various applications and modifications can be made,
without departing from the scope and spirit of the invention based
on the description above.
Therefore, the substantial scope of the present invention will be
defined by the accompanying claims and their equivalents.
REFERENCE NUMERALS
50: First adhesive layer 70: Third adhesive layer 80: Fourth
adhesive layer 90: Second adhesive layer 100: Display panel 105:
Panel substrate 110: Active layer 115: Gate insulating film 120:
Gate electrode 125: Interlayer insulating film 132: Source
electrode 134: Drain electrode 140: Via insulating film 145: Pixel
electrode 150: Pixel defining film 160: Display layer 170: Counter
electrode 180: Encapsulation layer 200: Functional structure 200a:
Touch sensor 200b: Polarizing layer 205: Substrate 210: Sensing
electrode 210a: First sensing electrode 210b: Second sensing
electrode 220: Trace 230: Pad 300: Window 350: Protective film
* * * * *